IGCSE Biology Paper 4 (Theory / Extended)

Topic 14: Coordination and Response -- Challenge Exam 3

Duration1 hour 15 minutes

Total Marks80

Questions7

Time Remaining75:00

Syllabus0610

Instructions

Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Your answers will be automatically graded when you submit.

Question Navigation

This paper covers the whole of Topic 14. Like a real Cambridge paper it ranges across every sub-topic — 14.1 neurones, the reflex arc and synapses, 14.2 sense organs and the eye, 14.3 hormones, 14.4 homeostasis, and 14.5 tropic responses and auxin — and it mixes them inside single questions. All three Topic 14 papers do; they differ in the angle they come at it from, not in what they cover.

Question 1 — A Cord Cut in Two Places

Total: 12 marks

Read all of this information before you begin. Everything that is new is given here.

In an accident a person’s spinal cord is damaged high up in the back. Below the level of the damage, the reflex arcs within the spinal cord are completely undamaged and so are all the neurones running to and from the skin and muscles of the legs. What has been destroyed are the neurones that carry impulses up the cord to the brain and down from the brain.

When the sole of the person’s foot is scratched, the foot moves away. The person reports feeling nothing at all in the leg, and cannot move the leg when asked to.

(a) [3]

Name, in order, the five parts of the pathway involved when the foot moves away.

Model Answer — 1(a)

receptor (in the skin of the foot) → sensory neurone [1]

→ relay neurone (in the spinal cord) [1]

→ motor neurone → effector (a muscle in the leg) [1]

⚠ If you missed marks here: The relay neurone is the part left out most often, and here it carries a mark of its own. Start at the stimulus and end at the response and you cannot accidentally stop early. Note that “effector” means a muscle or a gland, so naming the muscle as well as the term is worth doing.

1 mark — receptor then sensory neurone 1 mark — relay neurone in the spinal cord 1 mark — motor neurone then effector (muscle)

(b) [4]

Explain why the foot still moves away, but the person feels nothing and cannot move the leg voluntarily.

Model Answer — 1(b)

the reflex arc lies entirely within the spinal cord and does not involve the brain, so it is undamaged and still works [1]

the person feels nothing because the neurones carrying impulses up the cord to the brain have been destroyed, and the sensation of pain or touch is produced in the brain [1]

voluntary movement requires impulses from the brain to travel down the cord to the motor neurones, and that route is also destroyed [1]

this shows that the reflex response and the sensation are produced by two separate pathways, which is why one can work when the other does not [1]

⚠ If you missed marks here: This is the strongest possible evidence that a spinal reflex does not involve the brain, so any answer written from the “the brain decides to move the foot” picture collapses at the first line — if that were true, nothing would move at all here. Three separate things need explaining and each carries its own mark: why the reflex works, why sensation is absent, and why voluntary movement is absent. Answering only the first is the usual way to score one of four.

1 mark — the reflex arc is within the spinal cord and does not involve the brain, so it still works 1 mark — impulses cannot reach the brain, and sensation is produced in the brain 1 mark — voluntary movement needs impulses from the brain, and that route is destroyed 1 mark — the reflex and the sensation use two separate pathways

Describe what happens at a synapse in this reflex arc when an impulse arrives.

Model Answer — 1(c)

the impulse stimulates the release of neurotransmitter molecules from vesicles into the synaptic gap [1]

the molecules diffuse across the gap [1]

they bind with receptor proteins on the next neurone, which stimulates an impulse in it [1]

⚠ If you missed marks here: Nothing electrical crosses a synapse, so “the impulse jumps the gap” replaces all three marking points with nothing. The two verbs are diffuse and bind. It is worth noticing that this is the same process as diffusion in Topic 3, and it only works because the gap is a fraction of a micrometre wide — diffusion is fast over that distance and useless over any larger one.

1 mark — neurotransmitter released from vesicles into the synaptic gap 1 mark — molecules diffuse across the gap 1 mark — bind with receptor proteins, stimulating an impulse in the next neurone

(d) [2]

Suggest why this person is at particular risk of burning their legs on a hot surface without noticing.

Model Answer — 1(d)

although the reflex may move the leg, the person feels no pain, because no impulse reaches the brain [1]

so they do not know that contact has happened and cannot take voluntary action to move away or to check the injury; contact may therefore continue for a long time [1]

⚠ If you missed marks here: This part is asking you to see what pain is for. A reflex removes you from the stimulus once; the conscious sensation is what makes you check, treat and avoid it afterwards. An answer that says only “they cannot feel it” has the first mark; the second is for the consequence.

1 mark — no impulse reaches the brain, so no pain is felt 1 mark — no voluntary action can be taken, so contact and damage may continue

Question 2 — A Lens That Will Not Bend

Total: 12 marks

The nearest distance at which a person can focus clearly is called their near point. Table 2.1 shows the mean near point at different ages.

age / years	10	20	30	40	50	60
near point / cm	7	10	14	22	40	100

(a) [2]

Describe the trend shown in Table 2.1, and calculate the percentage increase in the near point between the ages of 40 and 60.

Model Answer — 2(a)

the near point increases with age, slowly at first and then far more steeply after about 40 (from 7 cm at age 10 to 100 cm at age 60) [1]

percentage increase = (100 − 22) ÷ 22 × 100 = 355 % (accept 354–355 %) [1]

⚠ If you missed marks here: Divide by the starting value, 22, not by the final value 100 — dividing by 100 gives 78 %, which is the answer to a different question and the one mark schemes watch for. For the description, “it goes up” is not enough: the interesting feature is that the increase is not steady, and saying where it accelerates is what earns the mark.

1 mark — the near point increases with age, accelerating after about 40 1 mark — percentage increase = 355 %

(b) [4]

Describe fully how a young person’s eye focuses on an object 10 cm away.

Model Answer — 2(b)

the ciliary muscle contracts, so the ring of muscle becomes narrower [1]

the suspensory ligaments slacken, so the lens is no longer pulled outwards [1]

the lens becomes fatter and more curved [1]

so it refracts the light more, bringing the strongly diverging rays from a near object to a focus on the retina [1]

⚠ If you missed marks here: Everything hangs on the first step: a ring of muscle contracting makes its own hole smaller, so the ligaments go slack. Write “contracts, so the ligaments tighten” and every later step is reversed with it. Two other refusals: the ligaments cannot contract, since they are not muscle, and the lens does not move, it changes shape.

1 mark — ciliary muscle contracts, ring becomes narrower 1 mark — suspensory ligaments slacken 1 mark — lens becomes fatter and more curved 1 mark — the lens refracts the light more, focusing it on the retina

With age the lens becomes harder and less elastic. Using this information, explain the trend in Table 2.1, and suggest why distant vision is much less affected.

Model Answer — 2(c)

the lens becomes fat by springing back elastically when the suspensory ligaments slacken — nothing pushes it into that shape [1]

a harder, less elastic lens cannot round itself up as much, so it cannot refract the light enough [1]

light from a near object is therefore brought to a focus behind the retina, so the object must be held further away to be seen clearly, and the near point increases [1]

distant vision is less affected because the thin shape is produced by the ligaments pulling on the lens, which still works on a hardened lens [1]

⚠ If you missed marks here: You have never been taught this condition, so the marks are entirely for the chain of reasoning from a single new fact. The step that unlocks it is realising that the fat shape depends on the lens’s own elasticity while the thin shape depends on something pulling — which is exactly why only one of the two fails. “The lens does not work as well” describes the outcome and explains nothing.

1 mark — the lens becomes fat by springing back elastically when the ligaments slacken 1 mark — a hardened lens cannot round up, so it cannot refract light enough 1 mark — light from near objects focuses behind the retina, so the near point increases 1 mark — distant vision survives because the ligaments still pull the lens thin

(d) [2]

A student says the data show that the ciliary muscle becomes weaker with age. Explain why the data do not show this.

Model Answer — 2(d)

the data show only that the near point increases; nothing about the ciliary muscle was measured [1]

the same result would be produced by a hardened lens with a perfectly normal ciliary muscle, so the two explanations cannot be distinguished by these measurements [1]

⚠ If you missed marks here: The skill being tested is spotting a claim about a structure in data that only recorded a result. That pattern recurs all through this topic — an auxin conclusion drawn from a bending seedling, an insulin conclusion drawn from a glucose graph. State what the data do show first, then give the alternative explanation, and say what would have to be measured to tell them apart.

1 mark — the data show only the near point; the ciliary muscle was not measured 1 mark — a hardened lens with a normal muscle would give the same result

Question 3 — A Liver That Cannot Store

Total: 12 marks

Read all of this information before you begin. Everything that is new is given here.

Some people are born with a liver that cannot convert glucose into glycogen, although it is normal in every other way. Their pancreas is completely normal and produces both insulin and glucagon in the usual amounts at the usual times.

Table 3.1 compares such a person, person V, with a healthy person, person U, after both drank the same glucose solution and then ate nothing for six hours.

time after drinking / hours	person U: blood glucose / units	person V: blood glucose / units
0	5	5
1	8	13
2	5	7
4	5	3
6	5	2

(a) [3]

Compare the results for person U and person V.

Model Answer — 3(a)

both start at the same value, 5 units [1]

V rises much higher than U after one hour (13 against 8 units) [1]

U returns to 5 units and stays there, whereas V continues to fall, reaching only 2 units at 6 hours [1]

⚠ If you missed marks here: V goes wrong in both directions, and most candidates spot only the high peak. The second half of the table — V falling to 2 units while U sits steady at 5 — is the part that parts (b) and (c) are built on. In a compare question, put both people in every sentence and attach a figure to each.

1 mark — both start at the same value, 5 units 1 mark — V rises much higher after one hour (13 against 8) 1 mark — U returns to and stays at 5, while V keeps falling to 2

(b) [3]

Explain why person V’s blood glucose concentration rises so much higher after the drink, even though their pancreas is normal.

Model Answer — 3(b)

the pancreas still detects the rise and still secretes insulin into the blood [1]

but the liver cannot convert the glucose into glycogen, so it cannot be stored [1]

the excess glucose therefore remains in the blood, and the concentration only falls as fast as glucose is taken up by muscle cells and used in respiration [1]

⚠ If you missed marks here: The whole question rests on separating the two organs. If you believed insulin itself converts glucose to glycogen, V’s result would be impossible — a normal pancreas would fix it. The result only makes sense if insulin is a signal and the liver does the work, which is exactly the distinction the paper is testing.

1 mark — the pancreas still detects the rise and secretes insulin 1 mark — the liver cannot convert glucose into glycogen for storage 1 mark — the excess glucose remains in the blood

Explain why person V’s blood glucose concentration continues to fall between 2 and 6 hours, and suggest one symptom they might experience.

Model Answer — 3(c)

glucose continues to be used by cells in respiration, and none is being absorbed from the gut [1]

the pancreas detects the fall and secretes glucagon [1]

but there is no glycogen stored in the liver to be converted back into glucose, so the correction cannot take place and the concentration keeps falling [1]

symptom: feeling faint, weak, dizzy or confused, because brain cells respire almost entirely using glucose and have too little available [1]

⚠ If you missed marks here: The neat point here is that one broken step ruins both halves of the loop: with no store, glucose cannot be put away when it is too high and cannot be released when it is too low. Say why glucagon fails rather than simply that it does — there is nothing left for it to act on. And attach a biological reason to the symptom; “they feel ill” is not worth a mark.

1 mark — glucose is still being used in respiration and none is absorbed from the gut 1 mark — the pancreas detects the fall and secretes glucagon 1 mark — there is no stored glycogen to convert back, so the correction fails 1 mark — faintness or weakness, because brain cells need glucose for respiration

(d) [2]

Explain why injecting insulin would not help person V, and suggest what management would be more useful.

Model Answer — 3(d)

V already produces normal amounts of insulin; the failure is in the liver, the target organ, so adding more of the signal cannot make a liver do something it is unable to do [1]

more useful: eat small meals frequently, including through the night, so that glucose is absorbed steadily and the concentration never rises far or falls far [1]

⚠ If you missed marks here: The general principle is worth carrying away: replacing a hormone only helps if the hormone is what is missing. In Type 1 diabetes it is, so injections work; here the signal is fine and the effector has failed. The suggestion in the second mark is genuine application, and any sensible reasoned answer that keeps the concentration steady would be credited.

1 mark — V already has normal insulin; the fault is in the liver, the target organ 1 mark — frequent small meals so glucose is absorbed steadily

Question 4 — Roots Down, Shoots Up, in the Dark

Total: 12 marks

Fig. 4.1 shows four germinating seeds. All four were kept in complete darkness at 20 °C for four days. Seeds 3 and 4 were pinned to a klinostat, a disc that rotates slowly about a horizontal axis.

(a) [3]

Define the term gravitropism, and describe the responses of the shoot and the root of seed 2 using the correct terms.

Model Answer — 4(a)

a response in which parts of a plant grow towards or away from gravity [1]

the shoot grows upwards, away from gravity: it is negatively gravitropic [1]

the root grows downwards, towards gravity: it is positively gravitropic [1]

⚠ If you missed marks here: The word grow is the mark in the definition — “the plant moves towards gravity” is refused, because a tropism is a growth response. Positive means towards the stimulus and negative means away from it, so the shoot growing up is negatively gravitropic, which sounds back to front until you remember that the stimulus points downwards.

1 mark — a growth response towards or away from gravity 1 mark — the shoot is negatively gravitropic 1 mark — the root is positively gravitropic

(b) [4]

Explain, in terms of auxin, why the shoot of seed 2 grew upwards.

Model Answer — 4(b)

auxin is made in the shoot tip and diffuses through the shoot [1]

it becomes unequally distributed in response to gravity, accumulating on the lower side [1]

auxin stimulates cell elongation, so the cells on the lower side become longer than those on the upper side [1]

the lower side therefore grows longer, so the shoot curves upwards, away from gravity [1]

⚠ If you missed marks here: The same four-step chain as for phototropism, with gravity in place of light and lower in place of shaded. Check it with the same trick: the shoot curves upwards, so the lower side must have grown more, so the auxin was on the lower side. Note that the syllabus asks for the auxin explanation for the shoot — roots respond differently and you are not asked to explain them chemically.

1 mark — auxin is made in the shoot tip and diffuses through the shoot 1 mark — it becomes unequally distributed, accumulating on the lower side 1 mark — auxin stimulates cell elongation, so lower-side cells become longer 1 mark — the lower side grows longer, so the shoot curves upwards

Explain the result for seed 3, and explain what the difference between seed 3 and seed 4 shows.

Model Answer — 4(c)

on the rotating klinostat, every side of the seedling spends equal time facing downwards, so the gravity stimulus is non-directional [1]

no unequal distribution of auxin builds up, so both sides elongate equally and the shoot and root grow straight [1]

seed 4 curved once the rotation stopped, which shows the seedling was still capable of responding all along — the klinostat removed the direction of the stimulus rather than damaging the seedling [1]

⚠ If you missed marks here: Do not write that the klinostat removes gravity — it cannot, and mark schemes draw that distinction explicitly. It makes the stimulus non-directional. Seed 4 is the control that most candidates ignore, and it does real work: without it you could argue that rotating the seedling had simply harmed it, and seed 4 rules that out.

1 mark — rotation makes gravity act equally on all sides, so the stimulus is non-directional 1 mark — no unequal auxin distribution, so growth is straight 1 mark — seed 4 curving after rotation stopped shows the seedling was still able to respond

(d) [2]

Suggest why it was important that all four seeds were kept in complete darkness.

Model Answer — 4(d)

light is a second directional stimulus that also causes an unequal distribution of auxin [1]

so if light were present it would not be possible to tell whether the curving was caused by gravity or by light; darkness leaves gravity as the only directional stimulus [1]

⚠ If you missed marks here: Say what would go wrong, not just “so light does not affect it”. The reasoning is the standard experimental-design point in a new costume: if two variables can both cause the effect, and both are allowed to vary, the experiment cannot tell you which was responsible. Note also that darkness does not stop the seedling growing — it is living on the food stored in the seed.

1 mark — light is a second directional stimulus affecting auxin distribution 1 mark — darkness leaves gravity as the only directional stimulus, so the cause is known

Question 5 — Two Systems, One Body

Total: 10 marks

(a) [4]

Compare nervous control and hormonal control in terms of speed of action and duration of effect, giving a reason for each difference.

Model Answer — 5(a)

nervous control acts much more quickly than hormonal control [1]

because impulses travel electrically along neurones that already reach the effector, whereas a hormone must be carried in the blood [1]

the effects of nervous control last a much shorter time than those of hormonal control [1]

because the effect ends almost as soon as the impulses stop, whereas a hormone continues to act until it is broken down [1]

⚠ If you missed marks here: A comparison needs both systems in the same sentence and a comparative word — faster, shorter, whereas. “Nervous is fast. Hormones are chemicals.” is two facts side by side and compares nothing. The syllabus limits this comparison to exactly two properties, so answer on those two and give a reason for each rather than listing everything you know.

1 mark — nervous control acts more quickly than hormonal control 1 mark — because impulses are electrical along neurones while hormones travel in the blood 1 mark — the effects of nervous control last a shorter time 1 mark — because a hormone continues to act until it is broken down

(b) [3]

For each of the following, state whether it is controlled by the nervous system or by a hormone, and justify your choice: (i) the pupil narrowing when a light is switched on, (ii) the changes in the body during puberty, (iii) the pupil widening when a person is frightened in a well-lit room.

Model Answer — 5(b)

(i) nervous — it happens within a fraction of a second and ends as soon as the light changes [1]

(ii) hormonal (oestrogen or testosterone) — the changes continue for years and must be sustained, which only a chemical circulating in the blood can achieve [1]

(iii) hormonal — caused by adrenaline, since the room is well lit and the light-driven reflex would be making the pupil smaller [1]

⚠ If you missed marks here: Parts (i) and (iii) describe the same organ doing opposite things for entirely different reasons, which is exactly what this question is checking. Read the stem for the cause: mention of light means the reflex, mention of fear means adrenaline. The justification is the mark — a bare choice with no reason earns nothing, and you have a fifty per cent chance of guessing it anyway.

1 mark — (i) nervous, because it is immediate and short-lived 1 mark — (ii) hormonal, because the changes are sustained over years 1 mark — (iii) hormonal (adrenaline), because the room is lit and fear is the cause

A person is told some frightening news over the telephone. Suggest a full pathway, from the stimulus to the secretion of adrenaline, using the terms stimulus, receptor, coordinator and effector.

Model Answer — 5(c)

stimulus = the sound of the words; receptor = receptor cells in the ear, which respond to sound [1]

impulses travel along a sensory neurone to the brain, which is the coordinator and interprets the meaning of the words [1]

the brain sends impulses to the adrenal glands, which are the effector — a gland — and they secrete adrenaline into the blood [1]

⚠ If you missed marks here: Two ideas make this answer work. First, an effector is a muscle or a gland, so a gland can perfectly well be the end of a nervous pathway — and here that is how the two systems join up. Second, the coordinator must be the brain, because the stimulus is the meaning of the words rather than the sound itself, and nothing in the spinal cord could do that.

1 mark — stimulus is sound; receptors are the sound receptor cells in the ear 1 mark — impulses travel along a sensory neurone to the brain, the coordinator 1 mark — the adrenal glands are the effector and secrete adrenaline into the blood

Question 6 — The Baby That Cannot Shiver

Total: 12 marks

Read all of this information before you begin. Everything that is new is given here.

A newborn baby cannot shiver. It also has a large surface area compared with its volume, and a thin layer of fatty tissue. Its other temperature control mechanisms work normally.

In an experiment, a newborn baby and an adult were each placed, lightly clothed, in a room at 20 °C for 30 minutes. Table 6.1 shows the results.

	newborn baby	adult
surface area to volume ratio	high	low
oxygen uptake at the start / cm³ per kg per minute	7.0	4.0
oxygen uptake after 30 min / cm³ per kg per minute	11.5	6.6
fall in core temperature / °C	0.9	0.1

(a) [3]

Calculate the percentage increase in oxygen uptake for each, and state what this shows about the rate of respiration.

Model Answer — 6(a)

baby: (11.5 − 7.0) ÷ 7.0 × 100 = 64 % [1]

adult: (6.6 − 4.0) ÷ 4.0 × 100 = 65 % [1]

oxygen is used in aerobic respiration, so both have increased their rate of respiration by a similar proportion in order to release more heat energy [1]

⚠ If you missed marks here: Divide by the starting value each time. The result is more interesting than it looks: the two percentages are almost identical, so the baby is not failing to increase its heat production — which rules out the obvious explanation for its larger temperature fall before you reach part (c). Always ask what a calculation has ruled out, not just what it has produced.

1 mark — baby: 64 % increase 1 mark — adult: 65 % increase 1 mark — oxygen uptake shows aerobic respiration, so both increased respiration similarly

(b) [3]

Explain how shivering raises body temperature, and suggest one other way the baby might increase its heat production without shivering.

Model Answer — 6(b)

shivering is rapid, involuntary contraction of muscles, which requires energy [1]

that energy comes from increased respiration in the muscle cells, and respiration also releases heat energy, warming the body [1]

the baby could increase its rate of respiration in other tissues, or move about and cry, which involves muscle contraction and therefore respiration [1]

⚠ If you missed marks here: Heat comes from respiration, not from friction — the friction answer sounds like physics and is simply not what happens. Keep the chain in order: contraction needs energy, energy comes from respiration, respiration releases heat. Part three is a “suggest”, so any sensible route to more respiration is credited, and the data support it: the baby’s oxygen uptake did rise 64 %, so it clearly found one.

1 mark — shivering is rapid involuntary muscle contraction requiring energy 1 mark — energy comes from increased respiration, which releases heat energy 1 mark — a sensible alternative route to increased respiration

Explain why the baby’s core temperature fell nine times as much as the adult’s, even though both increased their respiration by a similar proportion.

Model Answer — 6(c)

the baby has a high surface area to volume ratio, so it has much more surface per unit of body mass through which heat can be lost [1]

its layer of fatty tissue is thin, so there is less insulation and heat passes out more easily [1]

it cannot shiver, so it lacks the mechanism that produces the largest increase in heat production [1]

so although its heat production rose by a similar proportion, its heat loss was far greater, and the core temperature fell much further [1]

⚠ If you missed marks here: The last mark is the one that ties the answer together: temperature depends on the balance between heat produced and heat lost, and the calculation in part (a) has already told you production was not the problem. Answers that blame only the inability to shiver contradict their own arithmetic. The surface area to volume argument is the same one you used in Topic 11 for why a large organism needs lungs — a good example of one idea doing work in three different topics.

1 mark — high surface area to volume ratio means more surface for heat loss per unit mass 1 mark — thin fatty tissue means less insulation 1 mark — cannot shiver, so lacks the main heat-generating mechanism 1 mark — heat loss exceeded the increase in heat production, so the core temperature fell further

(d) [2]

Suggest two things that could be done to reduce the baby’s heat loss, and explain each.

Model Answer — 6(d)

wrap the baby in blankets or clothing, which traps a layer of air; still air is a poor conductor of heat, so it insulates [1]

raise the temperature of the room, which reduces the temperature difference between the baby and its surroundings and therefore reduces the rate of heat loss [1]

(also acceptable: cover the head, which is a large proportion of a newborn’s surface area)

⚠ If you missed marks here: Each mark needs a suggestion and its reason; a list of two sensible actions with no biology earns nothing. Notice that a blanket works by exactly the same mechanism as raised hairs — it traps air, and it is the air that insulates, not the fabric. That connection is worth making explicitly, because it shows you understand the mechanism rather than the example.

1 mark — wrap the baby, trapping a layer of air which is a poor conductor of heat 1 mark — warm the room, reducing the temperature difference and so the rate of heat loss

Question 7 — Six Sentences a Mark Scheme Would Refuse

Total: 10 marks

A student has written six sentences about Topic 14. Every one of them contains an error that would lose a mark.

(a) [3]

Correct these three sentences, explaining in each case what is wrong. (i) “When you touch something hot the impulse goes to the brain, which decides to move your hand.” (ii) “The impulse jumps across the synapse to the next neurone.” (iii) “The pupil controls how much light enters the eye.”

Model Answer — 7(a)

(i) a spinal reflex does not involve the brain: the impulse passes from the sensory neurone to a relay neurone in the spinal cord and out along the motor neurone; the brain receives a separate impulse afterwards, which is why the pain is felt second [1]

(ii) nothing electrical crosses a synapse: neurotransmitter is released from vesicles, diffuses across the gap and binds with receptor proteins, stimulating a new impulse in the second neurone [1]

(iii) the pupil is a hole and contains no muscle; it is the iris that controls how much light enters through the pupil [1]

⚠ If you missed marks here: Each mark requires the correction, not just the observation that something is wrong. These three are the most frequently penalised sentences in the whole topic, and they are worth reciting in their corrected form until they sound natural — because in an exam you will write whichever version comes to hand first.

1 mark — (i) corrected: the reflex passes through a relay neurone in the spinal cord, not the brain 1 mark — (ii) corrected: neurotransmitter diffuses across and binds with receptor proteins 1 mark — (iii) corrected: the iris controls how much light enters the pupil

(b) [3]

Correct these three sentences, explaining in each case what is wrong. (iv) “Insulin converts glucose into glycogen in the blood.” (v) “When you are hot, the blood vessels move up towards the surface of the skin.” (vi) “Auxin builds up on the lit side of the shoot and makes it bend towards the light.”

Model Answer — 7(b)

(iv) insulin is a hormone, not an enzyme, so it does not convert anything: it stimulates the liver to convert glucose into glycogen, and this happens inside the liver, not in the blood [1]

(v) blood vessels do not move; the arterioles supplying the surface capillaries dilate, so more blood flows through the capillaries near the surface and more heat is lost [1]

(vi) auxin accumulates on the shaded side, where it stimulates cell elongation; the shoot bends because that side grows longer, not because auxin bends it [1]

⚠ If you missed marks here: Each of these sentences contains two faults, and a full answer names both: (iv) wrong agent and wrong place; (v) wrong action and wrong vessel; (vi) wrong side and wrong mechanism. Spotting the second fault in each is what separates a good answer from a complete one, and it is exactly the habit that protects you when you write these ideas out under time pressure.

1 mark — (iv) corrected: insulin stimulates the liver, and the conversion happens in the liver 1 mark — (v) corrected: the arterioles dilate; vessels change diameter, not position 1 mark — (vi) corrected: auxin accumulates on the shaded side and stimulates cell elongation

Write a full answer to this question: “Explain how the body responds when the blood glucose concentration falls below the set point. [4]”

Model Answer — 7(c)

the fall is detected by the pancreas [1]

which secretes glucagon into the blood [1]

glucagon is carried to the liver, which it stimulates to convert stored glycogen back into glucose [1]

the glucose is released into the blood, so the concentration rises back towards the set point — the change has been reversed, which is negative feedback [1]

⚠ If you missed marks here: Four marks, four links, written in order — and you can count them as you go and know when you have finished. Check the two words that look alike: glucagon is the hormone from the pancreas and glycogen is the store in the liver, and swapping them turns a correct answer into a meaningless one. The final phrase, back towards the set point, is what makes it a control system rather than a one-way effect.

1 mark — the fall is detected by the pancreas 1 mark — glucagon is secreted into the blood 1 mark — the liver converts stored glycogen back into glucose 1 mark — glucose is released and the concentration rises back towards the set point

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